This invention relates to compositions useful as thickening agents in aqueous brine systems and to the aqueous well drilling and servicing fluids prepared therefrom.
In the drilling of subterranean wells, aqueous fluids containing no suspended solids have been used extensively in completion and workover operations in order to minimize damage to the production stratum caused primarily by fluid and solids invasion. After high solids drilling muds have been employed, these fluids are flushed through the well in order to remove any moveable solids which may cause clogging and in turn prevent oil from freely flowing into the producing zone. The fluids also function to control subsurface pressure and maintain hole stability while remaining stable at working temperatures. The fluids are concentrated water solutions of metal salts including, for example, KCl, NaCl, NaBr, CaCl.sub.2, CaBr.sub.2, ZnCl.sub.2 and ZnBr.sub.2 . The salt or salts selected and the concentration employed are determined by the desired density and crystallization temperature of the fluid. Modern research has led to the development of a number of useful brines with densities ranging up to 19.2 ppg. (pounds per gallon).
Additives are used in the brine solutions to control and increase their viscosity which in turn enhances the cuttings carrying capacity of the brines, reduces fluid loss, and conditions and minimizes caving and water damage of the water-sensitive formation. Polysaccharide gums such as xanthan gum, guar gum, locust bean gum, carrageenan, and hydroxyethyl cellulose (HEC) have been employed to thicken brine systems. These additives, normally supplied in dry powder form, require special preparation and/or mixing and dispersing equipment when added to the brine systems. HEC and other polysaccharide additives surface-hydrate too rapidly and nonuniformly, resulting in clumps referred to as "fish eyes" which can cause clogging and formation damage. Many of these polysaccharide additives hydrate only after elevating the temperature of the brine. As these additives dissolve upon exposure to higher downhole temperatures, they cause brine viscosities to fluctuate resulting in unpredictable and undesirable latent viscosities. Moreover, many of the polysaccharides tend to also form separate gummy polymer layers in the higher density brines.
Various attempts have been made involving additional time and expense to overcome the dissolution problems described above. For example, in U.S. Pat. No. 4,392,964 issued on July 12, 1983 to R. House et al., a polymer composition prepared by mixing 5-30 parts of HEC with at least 40 parts of isopropanol is mixed with 3-40 parts water in order to thoroughly wet the HEC prior to addition to the brine system. In U.S. Pat. No. 4,415,463 issued on Nov. 15, 1983 to B. Mosier et al., natural polysaccharide gums such as carageenan and locust bean are treated with a basic nitrogen reagent to wet the polymer prior to addition to the brine system. In U.S. Pat. Nos. 4,435,564 and 4,439,333 issued on Mar. 6, 1984 and Mar. 27, 1984 respectively to R. House, HEC is first activated such that the HEC will disperse and hydrate in heavy brines. The activation process comprises admixing HEC in a solution of either an amino or phenolic compound and a water soluble organic liquid prior to brine addition. Some of the additives also become ineffective when added to brine systems having densities above about 12 ppg. In U.S. Pat. No. 4,420,406 issued on Dec. 13, 1983 to R. House et al. it was disclosed that heavy brine solutions having a narrow density range of 14.2 to 15.6 ppg could be effectively thickened with HEC depending on the presence or absence of CaCl.sub.2 and the specific amount of ZnBr.sub.2 in the brine.
Other problems still exist when the polysaccharides described above are added to brine solutions. Many of the polysaccharides are subject to incompatibility problems in the brines after a period of time. The polysaccharides also become thermally unstable when subjected to downhole temperatures above about 180.degree. F. (82.degree. C.). Furthermore some of the polysaccharides employed are subject to bacterial attack. For these reasons the subterranean well drilling industry is searching for other means to effectively thicken brine solutions.
It is therefore an object of the present invention to provide brine thickeners which will easily dissolve, be compatible in the brines, be thermally resistant, and not be subject to bacterial attack.